Color photographic materials employing two-equivalent pyrazolone magenta dye-forming couplers are known in the art as demonstrated, for example, by the Sakai et al U.S. Pat. No. 4,483,918, the Furutachi et al U.S. Pat. No. 4,585,728 and German Off. DE 3,730,557. Two-equivalent pyrazolone magenta couplers are advantageous for use in color photographic materials owing to their low cost, high efficiency, good activity, adjustable hue and suitability for use in processes without formaldehyde.
It is also well known in the color photographic art that couplers are used in combination with solvents and other addenda which facilitate their incorporation in the photographic materials and/or improve one or more properties of the dyes formed from the couplers. For example, the Ogawa et al U.S. Pat. No. 4,857,449 discloses combinations of couplers and one or more high boiling organic solvents for use in color photographic materials. The Sakai et al and Furutachi et al patents cited above and the Sakai et al U.S. Pat. No. 4,555,479 disclose the use of aniline and amine addenda with two-equivalent pyrazolone magenta couplers to reduce stain that occurs in development processing. The Kato et al. U.S. Pat. No. 4,171,975 discloses combinations of aldehydebis type magenta couplers in combination with a carbonamide compound.
One disadvantage associated with the two-equivalent pyrazolone magenta dye-forming couplers is that they have low pKa values. The pKa value is -log Ka, wherein Ka is the acid dissociation constant. Since these couplers tend to have low pKa values, they may be significantly ionized when films or papers coated with them are placed in solutions of low pH, i.e., a pH of 5-6, or less. Thus, when photographic materials containing these low pKa couplers are used in a process which does not employ a stop bath between the development and bleach steps, non-imagewise dye formation occurs owing to coupling with developer that is carried over into the bleach solution and oxidized therein. This phenomenon, which is referred to as continued coupling, produces undesirable increases in background density (Dmin). Continued coupling also leads to unacceptable density variability in processed films owing to variations in bleach pH as the bleach solutions become "seasoned" by continued use. Accordingly, photographic films and papers containing low pKa couplers such as the two-equivalent pyrazolone couplers often exhibit continued coupling because the couplers are more highly ionized at low pH and thus readily react with oxidized developer in the low pH bleach solutions. Thus, there is a need to provide color photographic materials which contain two-equivalent pyrazolone magenta dye-forming couplers and which exhibit a reduction in the continued coupling phenomenon.